Overview

The CE8 is a current-excitation module that applies a constant current to the heating element of up to eight 229 Heat Dissipation Water Matric Potential Sensors. A current-excitation module is required when you use 229 sensors to measure soil water matric potential.

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Detailed Description

A CE8 current excitation module can provide a constant current to the heating element of the 229. The CE8 sources current for up to eight 229s. The module requires a 12 Vdc power source. The number of 229 sensors measured by one datalogger can be increased by connecting a CE8 to one or more AM16/32-series multiplexers. Up to eight multiplexers can be connected to the CE8.

Note: The CE8 module switches currents that are greater than 30 mA, which degrades the contact surfaces of the mechanical relays. Therefore, the multiplexer channels to which the CE8 have been connected may become unsuitable for other applications. (Refer to the multiplexer manual for more information.)

Specifications

Output

50 mA ±0.25 mA (regulated)

Output Channels

8

Power

12 Vdc source required

Active Current Drain

25 mA + (50 mA) x (number of 229s connected to the CE8)

Compliance

CE compliant

Dimensions

16.5 x 5.4 x 2.7 cm (6.5 x 2.1 x 1.1 in.)

Weight

184 g (6.5 oz)

Compatibility

Datalogger Considerations

The CABLE3CBL can be used to connect the CE8 to the datalogger. (See the Ordering information.)

Compatible Contemporary Dataloggers

CR200(X) Series

CR800/CR850

CR1000

CR3000

CR9000X

Compatible Retired Dataloggers

CR500

CR510

CR10

CR10X

21X

CR23X

CR9000

CR5000

CR7X

Power Considerations

The CE8 requires 12 Vdc power. A datalogger control port is typically used to activate the module. When the control signal is received, all 8 channels output a 50 mA source current to the connected loads (typically 229 probes). When the control signal is dropped to 0 Vdc, output from the module ceases.

When active, the current required from the 12 V supply is approximately the following:

Multiplexers

The CE8 can source current for eight probes directly. In applications that require more sensors, the output(s) of the CE8 can be connected to as many as eight AM16/32B multiplexers, greatly expanding system capacity.

When using multiplexers, the user should be aware that switching currents of greater than 30 mA will degrade the contact surfaces of the mechanical relays. This will adversely affect the suitability of these relays to multiplex low voltage signals. Although a relay used in this manner no longer qualifies for low voltage measurements, it continues to be useful for switching currents in excess of 30 mA. Therefore the user is advised to record which multiplexer channels are used to multiplex the 50 mA excitation for the 229-L probes in order to avoid using those channels for low voltage measurements in future applications.

Enclosure Considerations

The CE8 requires a desiccated, non-condensing environment; a Campbell Scientific enclosure is recommended. The CE8's case has mounting flanges for attachment to the 1"-on-center hole grid of Campbell Scientific enclosures. Grommets and screws are provided to attach the flanges to the backplate of our enclosures.

While this could be done for a maximum resistance of 50 ohms in the ±2500 mV range and 100 ohms in the ±5000 mV range, it is not the best way to determine resistance. The preferred method for measuring an unknown resistance is to use datalogger excitation channels. Current excitation channels can be used with a CR3000. Analog excitation channels with half-bridge or full-bridge circuitry can be used for other dataloggers. See the specific datalogger’s manual for further details

Campbell Scientific recommends using the CABLE3CBL-L, a 22 AWG cable with three conductors and a drain wire. However, any type of copper wire between 18 AWG and 22 AWG could be used to control the CE4/CE8.